Vibrational adsorption spectra

This distance shrinks as v increases, until it finally disappears entirely. Thus, the vibrational energy cannot surpass a certain limit. If we give the molecule more energy than that limit, it dissociates. This results in the presence of a continuous adsorption spectrum beyond a certain value vum of v, so that the difference = 0, which gives us the value ... 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

RAIRS spectra contain absorption band structures related to electronic transitions and vibrations of the bulk, the surface, or adsorbed molecules. In reflectance spectroscopy the ahsorhance is usually determined hy calculating -log(Rs/Ro), where Rs represents the reflectance from the adsorhate-covered substrate and Rq is the reflectance from the bare substrate. For thin films with strong dipole oscillators, the Berre-man effect, which can lead to an additional feature in the reflectance spectrum, must also be considered (Sect. 4.9 Ellipsometry). The frequencies, intensities, full widths at half maximum, and band line-shapes in the absorption spectrum yield information about adsorption states, chemical environment, ordering effects, and vibrational coupling. 

Figure 11. Infrared spectrum of CO adsorption at 295K for the Pt/SBA-15 catalyst series (a) 2.33% Pt(1.7nm)/SBA-15, (b) 2.69% Pt(2.9 nm)/SBA-15, (c) 2.62% Pt(3.6nm)/SBA-15, and (d) 2.86% Pt(7.1 nm)/SBA-15. Inset is the peak position and FWHM of the atop CO stretching vibration as a function of particle size at room temperature. Peak heights have been modified for clarity [16]. (Reprinted from Ref [16], 2006, with permission from American Chemical Society.)...

Thomas GE, Weinberg WH. 1979. The vibrational spectrum and adsorption site of CO on the Ru(OOOl) surface. J Chem Phys 70 1437. 

The development of the theory of the rate of electrode reactions (i.e. formulation of a dependence between the rate constants A a and kc and the physical parameters of the system) for the general case is a difficult quantum-mechanical problem, even when adsorption does not occur. It would be necessary to consider the vibrational spectrum of the solvation shell and its vicinity and quantum-mechanical interactions between the reacting particles and the electron at various energy levels in the electrode. 

Here it is our intention to show that for a system constituted by substrate phonons and laterally interacting low-frequency adsorbate vibrations which are harmonically coupled with the substrate, the states can be subclassified into independent groups by die wave vector K referring to the first Brillouin zone of the adsorbate lattice.138 As the phonon state density of a substrate many-fold exceeds the vibrational mode density of an adsorbate, for each adsorption mode there is a quasicontinuous phonon spectrum in every group of states determined by K (see Fig. 4.1). Consequently, we can regard the low-frequency collectivized mode of the adsorbate, t /(K), as a resonance vibration with the renormalized frequency and the reciprocal lifetime 7k-... 

Figure 4. Vibrational spectra of NA. Experimental conditions adsorption from 1 mM NA in 10 mM KF, pH 3 (A and B) or pH 7 (C), followed by rinsing with 2 mM HF (A and B, pH 3) or 0.1 mM KOH (pH 10 for C) EELS incidence and detection angle 62 from the surface normal beam energy, 4 eV beam current about 120 pA EELS resolution, 10 meV (80 cm-1) F.W.H.M. IR resolution, 4 cm-1. A. Upper curve EELS spectrum of NA adsorbed at Pl(lll) [pH 3 electrode potential, -0.3 V]. Lower curve in A and B mid-IR spectrum of NA vapor (18). Continued on next page.

Figure 8. Vibrational spectra of pyridine. Upper curve EELS spectrum of PYR adsorbed at Pt(lll) (pH 3) lower curve mid-IR spectrum of liquid PYR (18). Experimental conditions adsorption at -0.1 V from 1 mM PYR in 10 mM KF (pH 3), followed by rinsing with 2 mM HF (pH 3) other conditions as in Figure 4.

Pyridine. Pyridine and its methyl substituted derivatives (picolines and lutidines) were found to polymerize electrochemically and, under certain circumstances, catalytically. This behavior was not expected because usually pyridine undergoes electrophilic substitution and addition slowly, behaving like a deactivated benzene ring. The interaction of pyridine with a Ni(100) surface did not indicate any catalytic polymerization. Adsorption of pyridine below 200 K resulted in pyridine adsorbing with the ring parallel to the surface. The infrared spectrum of pyridine adsorbed at 200 K showed no evidence of either ring vibrations or CH stretches (Figure 5). Desorption of molecular pyridine occurred at 250 K, and above 300 K pyridine underwent a... 

For this case, the primary change that is observable in the IR spectrum is due to changes in the vibrahonal frequencies of the probe molecule due to modificahons in bond energies. This can lead to changes in bond force constants and the normal mode frequencies of the probe molecule. In some cases, where the symmetry of the molecule is perturbed, un-allowed vibrational modes in the unperturbed molecule can be come allowed and therefore observed. A good example of this effect is with the adsorption of homonuclear diatomic molecules, such as N2 and H2 (see Section 4.5.6.8). 

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